1. Field of the Invention
This invention relates to a solid electrolyte cell in which a solid electrolyte is impregnated in a positive electrode active material or in a negative electrode active material, and a method for producing the cell.
2. Description of the Related Art
Recently, many types of portable electronic equipments, such as a video tape recorder with a built-in camera, portable telephone or a portable computer, have made their debut, and attempts are being made for reducing their size and weight. Also, researches into cells as portable power sources of these electronic equipments, in particular the secondary cells, are proceeding. Of lithium cells, among these secondary cells, researches and investigations into the thin type cell or foldable cells are proceeding most briskly. As the electrolytes for these cells, investigations into the solid electrolyte obtained on solidifying the electrolyte, are proceeding energetically. In particular, high molecular solid electrolyte having lithium salts dissolved in the high-molecular material or the gelated solid electrolyte containing a plasticizer, are stirring up notice.
With the solid electrolyte, the cell can be reduced in thickness more significantly than with the liquid electrolyte, while there is no risk of leakage of the cell contents. However, if the solid electrolyte is used in a cell, it is not fluid as is the liquid electrolyte, so that it can hardly be contacted in an ideal state with the electrode. Since ions are migrated in the cell through the solid electrolyte or the gelated electrode, the contacting state between the solid electrolyte and the electrode affects the cell performance significantly. If the contacting state between the solid electrolyte and the electrode is poor, the contact resistance between the solid electrolyte and the electrode is increased to increase the internal resistance of the cell. Moreover, ions cannot be migrated in an ideal state between the solid electrolyte and the electrode to decrease the cell capacity. It is therefore crucial for the solid electrolyte to have a sufficiently tight contact with a layer of the active material of the electrode.
It is reported in Japanese Laying-Open Patent H-2-40867 to use a positive electrode composite, obtained on adding a solid electrolyte to the layer of the active material of the positive electrode to improve the contact state between the solid electrolyte and the electrode. In the cell disclosed in this publication, a portion of the solid electrolyte is mixed to a layer of the active material of the positive electrode to improve the state of electric contact between the solid electrolyte and the electrode.
With the cell disclosed in this publication, since the positive electrode plate is fabricated using a positive electrode composite admixed with the solid electrolyte, and a solid electrolyte is layered on the positive electrode plate, it is difficult to realize an ideal contact state between the positive electrode plate and the solid electrolyte. In particular, if the solid electrolyte having a roughed surface is layered on the electrode layer, the two are bonded only in a poor contact state to increase the internal resistance, thus worsening the load characteristics.
Moreover, the positive electrode composite admixed with the solid electrolyte, or the negative electrode composite, cannot be pressed sufficiently because of the elasticity proper to the solid electrolyte to increase the inter-particulate distance in the composite and hence the internal resistance, thus again worsening the load characteristics.
In addition, the positive electrode composite admixed with the solid electrolyte, or the negative electrode composite, need to be fabricated at low humidity to prevent decomposition of the electrolytic salt contained in the solid electrolyte. This not only raises difficulties in quality control but increases the cost significantly.
It is therefore an object of the present invention to provide a solid electrolyte cell in which the electrical contact state between the solid electrolyte and the layers of the active materials of the positive and negative electrodes and the inter-particulate distance in the layers of the active materials of the positive and negative electrodes can be optimized to assure superior load characteristics, and a method for producing the solid electrolyte cell.
In one aspect, the present invention provides a solid electrolyte cell including an electrode having a current collector and a layer of an active material formed on the current collector and containing an active material and a binder, in which a solid electrolyte layer is formed by impregnating a solid electrolyte dissolved in a solvent into the layer of the active material.
In the solid electrolyte cell of the present invention, since the solid electrolyte is impregnated in the layer of the active material, adhesion between the electrolyte and the active material is improved.
In another aspect, the present invention provides a method for manufacturing a solid electrolyte cell including the steps of applying a paint containing an active material and a binder to a current collector to form a layer of an active material, and impregnating a solid electrolyte in the layer of the active material formed by the active material layer forming step. The impregnating step includes applying the paint comprised of the solid electrolyte dissolved in a solvent on the layer of the active material to allow the paint to be permeated into the layer of the active material and subsequently drying the solvent.
In the manufacturing method of the solid electrolyte cell according to the present invention, the paint containing the active material and the binder is coated on the current collector to form a layer of the active material during the active material layer forming step. In the impregnating step, the paint comprised of the solid electrolyte dissolved in the solvent is applied on the layer of the active material to allow the paint to be permeated into the active material layer. The solvent is then dried to impregnate the solid electrolyte in the active material layer. In the manufacturing method of the solid electrolyte cell according to the present invention, the adhesion between the solid electrolyte and the active material is improved by impregnation of the solid electrolyte into the layer of the active material.
Thus, the contacting performance between the active material ands the electrolyte is improved to reduce the internal resistance to achieve a cell having superior load characteristics.